During the drilling process it is necessary for large amounts of drilling fluid to be pumped down the hole in order for the velocity of the fluid in the annulus to be of sufficient velocity to transport the cuttings that are generated at the bit to the surface. These circulating rates are typically in the order of 1-4 m3/min. As is known, this amount of fluid must be rapidly processed over a primary solids removal system which is typically one or more shakers configured to receive the recovered drilling fluid and cuttings. A shaker includes a plurality of screens that are actively vibrated so as to encourage the liquid components to pass through the screen and the solid components (i.e. drill cuttings) to be recovered from the top-side of the screen. Shakers generally have a relatively small surface area for the volume of recovered fluids and, as such, must be vibrated at a high rate to provide effective separation.
Cyclones (or cyclone separators) are typically used in the hydrocarbon industry to remove mist or small particles from gas streams by rapidly circulating the gas stream around a circular body to impart a centrifugal force on the particles to effect separation from the gas. Dust removal is a common application. Similarly, a hydrocyclone is used to separate particles entrained in a liquid and a centrifugal decanter is used to separate two liquids. The general operation of each device is described in relation to a cyclone.
Cyclones are typically conical in shape wherein input fluids (gases and liquids with any entrained solids) enter near the top through a tangential nozzle and move in a spiral. As the fluids move, entrained particles are impinged against the wall where they slide down due to gravity for collection, while the gas escapes through the top of the device.
The separation factor of a cyclone is defined as the ratio of centrifugal to gravitational forces:
      Separation    ⁢                  ⁢    Factor    =                    F        centrifugal                    F                  gravity          ⁢                                                      =                                        m            ⁢                                                  ⁢                          u              tan              2                                            r            ⁢                                                  ⁢                          g              c                                                            m            ⁢                                                  ⁢            g                                g            c                              =                        u          tan          2                          r          ⁢                                          ⁢          g                    
In most cyclones the particles being separated are small enough that Stokes' Law can be used to determine the drag force. This means that the force balance on a particle under centrifugal force becomes
                    (                                            π              ⁢                                                          ⁢                              d                p                3                                      6                    ⁢                      ρ            p                          )            ⁢                        ⅆ          v                          ⅆ          t                      =                                        (                                                            π                  ⁢                                                                          ⁢                                      d                    p                    3                                                  6                            ⁢                              ρ                p                                      )                    ⁢                                                    r                ⁢                                                                  ⁢                                  ω                  2                                            ⁢                                                                                  g              c                                      -                              3            ⁢            π            ⁢                                                  ⁢            μ            ⁢                                                  ⁢                          d              p                        ⁢                          v              radial                                            g            c                          -                              (                                                            π                  ⁢                                                                          ⁢                                      d                    p                    3                                                  6                            ⁢              ρ                        )                    ⁢                                    r              ⁢                                                          ⁢                              ω                2                                                    g              c                                          =                                                                  πd                p                3                            ⁢              r              ⁢                                                          ⁢                              ω                2                                                    6              ⁢                                                          ⁢                              g                c                                              ⁢                      (                                          ρ                p                            -              ρ                        )                          -                              3            ⁢            π            ⁢                                                  ⁢            u            ⁢                                                  ⁢                          d              p                        ⁢                          v              radial                                            g            c                                ⁢        
Since the acceleration phase for the moving particle is fairly brief, the velocity can be treated as constant with respect to time (though not with respect to position) and the force balance solved for the radial velocity
      0    =                                        π            ⁢                                                  ⁢                          d              p              3                        ⁢            r            ⁢                                                  ⁢                          ω              2                                            6            ⁢                          g              c                                      ⁢                  (                                    ρ              p                        -            ρ                    )                    -                        3          ⁢          π          ⁢                                          ⁢          μ          ⁢                                          ⁢                      d            p                    ⁢                      v            rad                                    g          c                                v      rad        =                                        π            ⁢                                                  ⁢                          d              p              3                        ⁢            r            ⁢                                                  ⁢                                          ω                2                            ⁡                              (                                                      ρ                    p                                    -                  ρ                                )                                                          6            ⁢                                                  ⁢                          g              c                                      ⁢                                  ⁢                              g            c                                3            ⁢            π            ⁢                                                  ⁢            u            ⁢                                                  ⁢                          d              p                                          =                                                  d              p              2                        ⁡                          (                                                ρ                  p                                -                ρ                            )                                ⁢                                          ⁢          r          ⁢                                          ⁢                      ω            2                                    18          ⁢          μ                    
which can in turn be expressed in terms of the gravitational terminal velocity and the tangential velocity
      v    rad    =                    (                                                            d                p                2                            ⁡                              (                                                      ρ                    p                                    -                  ρ                                )                                                    18              ⁢              μ                                ⁢                      g            g                          )            ⁢              (                  r          ⁢                                          ⁢                      ω            2                          )              =                            μ          i                g            ⁢                        μ          tan          2                r            
From these equations, it can be seen that the higher the terminal velocity, the higher the radial velocity, and thus the easier the separation.
In the foregoing: g=gravitational constant 981.65 cm/s2 9.81 m/s2; r=radius of rotation (m); ω=angular velocity in radians s−1; μ=dynamic viscosity (Pa·s); dp=particle diameter (m); ρp=particle density (kg/m3) and, ρ=fluid/gas density (kg/m3).
A review of the prior art reveals that cyclones have been used in various systems in the past. For example, U.S. Pat. No. 4,279,743 utilizes injected air bubbles to cause the boundary layer on the wall of the cyclone to be disrupted causing solid particles to separate from the fluid carrier; U.S. Pat. No. 4,971,685 uses injected air to create a froth in the hydrophobic part of the slurry allowing it to be recovered and the hydrophilic portion discharges; U.S. Pat. No. 6,155,429 injects air at very low concentrations and rates relative to the liquid phase(s); U.S. Pat. No. 7,841,477 and U.S. Pat. No. 4,764,287 are two phase cyclones; U.S. Pat. No. 6,348,087 is a three phase cyclone separator that requires three chambers; and, U.S. Pat. No. 5,332,500 describes a three phase cyclone designed to separate two fluids with different densities.